Nanoscale Ripple Research Articles

Dependence of capillary forces on relative humidity and the surface properties of femtosecond laser micromachined titanium

Capillary forces were measured with colloidal atomic force microscopy at different levels of relative humidity on femtosecond laser micromachined titanium surfaces. After laser machining at different intensity levels, the titanium surfaces show a nanoscale ripple topology or microscopic bumpy structures. Different machining environments were chosen to influence the surface chemistry in addition to topology: while machining in pure oxygen and water resulted in surfaces consisting of TiO2, a composite surface of TiO2 and TiN was obtained after machining in pure nitrogen. All samples were subsequently exposed to pure oxygen, carbon dioxide or water, and showed different levels of wettability and capillary force. We have introduced the concept of humidity sensitivity as the relative increase of the capillary force with respect to the measured force at 0% humidity. We report that samples with a nanoscale ripple topology machined in pure oxygen exhibit the lowest level of capillary force and the lowest sensitivity towards humidity in the environment. Surfaces with low sensitivity towards changes of the relative humidity are good candidates for technical applications, where capillary forces have to be controlled. This study contributes to the development of such surfaces, to a better understanding of how capillary bridges are formed on rough surfaces and ultimately to the exploration of the relationship between surface wettability and capillary forces.

Flipping nanoscale ripples of free-standing graphene using a scanning tunneling microscope tip

We present a scanning tunneling microscopy study of the stress–strain behavior of a rippled single-layer free-standing graphene (FSG) and report that FSG exhibits a non-linear sigmoidal stress–strain behavior. We managed to pull and push nanoripples by controlling the STM tip–FSG interaction forces. In this way, we found that the initial deformations of a rippled FSG involve sign reversals of the nanoripple’s local curvatures, termed “flipping”. In contrast to elastic deformation of graphene, flipping of a FSG nanoripple encounters a stress barrier and therefore makes a rippled FSG metastable, as evidenced by monitoring a yielding process in both experiments and molecular dynamics simulations. The evolution of nanoripples subjected to STM control is also fully consistent with atom-resolved images. Our work therefore shows how rippled 2D carbon deforms at nanoscale.

On the limits of uniaxial magnetic anisotropy tuning by a ripple surface pattern

Ion beam patterning of a nanoscale ripple surface has emerged as a versatile method of imprinting uniaxial magnetic anisotropy (UMA) on a desired in-plane direction in magnetic films. In the case of ripple patterned thick films, dipolar interactions around the top and/or bottom interfaces are generally assumed to drive this effect following Schlömann's calculations for demagnetizing fields of an ideally sinusoidal surface [E. Schlömann, J. Appl. Phys. 41, 1617 (1970)]. We have explored the validity of his predictions and the limits of ion beam sputtering to induce UMA in a ferromagnetic system where other relevant sources of magnetic anisotropy are neglected: ripple films not displaying any evidence of volume uniaxial anisotropy and where magnetocrystalline contributions average out in a fine grain polycrystal structure. To this purpose, the surface of 100 nm cobalt films grown on flat substrates has been irradiated at fixed ion energy, fixed ion fluency but different ion densities to make the ripple pattern at the top surface with wavelength Λ and selected, large amplitudes (ω) up to 20 nm so that stray dipolar fields are enhanced, while the residual film thickness t = 35–50 nm is sufficiently large to preserve the continuous morphology in most cases. The film-substrate interface has been studied with X-ray photoemission spectroscopy depth profiles and is found that there is a graded silicon-rich cobalt silicide, presumably formed during the film growth. This graded interface is of uncertain small thickness but the range of compositions clearly makes it a magnetically dead layer. On the other hand, the ripple surface rules both the magnetic coercivity and the uniaxial anisotropy as these are found to correlate with the pattern dimensions. Remarkably, the saturation fields in the hard axis of uniaxial continuous films are measured up to values as high as 0.80 kG and obey a linear dependence on the parameter ω2/Λ/t in quantitative agreement with Schlömann's prediction for a surface anisotropy entirely ruled by dipolar interaction. The limits of UMA tuning by a ripple pattern are discussed in terms of the surface local angle with respect to the mean surface and of the onset of ripple detachment.

Soft picosecond X-ray laser nanomodification of gold and aluminum surfaces

Experimental and theoretical investigations of aluminum (Al) and gold (Au) surface modification by soft X-ray laser pulse are presented. Well-polished samples of Al and Au are irradiated by ps-duration pulse with wavelength of 13.9 nm at the energy range of 24–72 nJ. Differences in the melting and the ablation processes for those materials are observed. It is shown that at low laser pulse energy, the nanoscale ripples on the surface may be induced by melting without following ablation. In that case, the nanoscale changes in the surface are caused by splash of molten metal under gradient of fluence. At higher laser pulse energy, the ablation process occurs and craters are formed on the surface. However, the melting determines the size of the modified surface at all ranges of the laser energies. For interpretation of experimental results, the atomistic simulations of melting and ablation processes in Al and Au are provided. The calculated threshold fluencies for melting and ablation are well consistent with measured ones.

Simulation of Nanopattern Formation of Si Surface due to Ion Irradiation

Energetic ion irradiation induce surface nano-scale patterns formation, ripple or wavelike
 pattern is one of the surface morphologies. Analytical solution of the BH continuum equation, using
 MATLAB applied on Si target, well-defined ripple structure were observed as a function of
 different parameters such as angle of ion incidence, ion energy, ion flux, ion species, and thermal
 diffusion coefficient

Topography-controlled alignment of DNA origami nanotubes on nanopatterned surfaces

The controlled positioning of DNA nanostructures on technologically relevant surfaces represents a major goal along the route toward the full-scale integration of DNA-based materials into nanoelectronic and sensor devices. Previous attempts to arrange DNA nanostructures into defined arrays mostly relied on top-down lithographic patterning techniques combined with chemical surface functionalization. Here we combine two bottom-up techniques for nanostructure fabrication, i.e., self-organized nanopattern formation and DNA origami self-assembly, in order to demonstrate the electrostatic self-alignment of DNA nanotubes on topographically patterned silicon surfaces. Self-organized nanoscale ripple patterns with periodicities ranging from 20 nm to 50 nm are fabricated by low-energy ion irradiation and serve as substrates for DNA origami adsorption. Electrostatic interactions with the charged surface oxide during adsorption direct the DNA origami nanotubes to the ripple valleys and align them parallel to the ripples. By optimizing the pattern dimensions and the Debye length of the adsorption buffer, we obtain an alignment yield of ∼70%. Since this novel and versatile approach does not rely on any chemical functionalization of the surface or the DNA nanotubes, it can be applied to virtually any substrate material and any top-down or bottom-up nanopatterning technique. This technique thus may enable the wafer-scale fabrication of ordered arrays of functional DNA-based nanowires.

Effect of energy density on the machining character of C/SiC composites by picosecond laser

The effect of laser energy density on the machining character of C/SiC composites by picosecond laser was investigated using two machining modes: single ring line and helical lines scanning. For single ring line scanning mode, the width and depth of machining grooves increased nonlinearly with the increase of laser energy density. Moreover, periodic surface structures (ripples) were generated at relative low laser energy density and disappeared at high energy density. With the increasing energy density, the oxygen content of machining debris increased dramatically. For helical lines scanning mode, the depth of machining grooves increased nonlinearly with the increasing laser energy density. With the increasing energy density, the oxygen content of machining debris also increased dramatically. The machining character showed as nano-scale laser-induced ripples, pores, strip structures and bubble pits. Finally, micro-holes of high aspect ratio were obtained in the mode of helical lines scanning by removing multiple layers.

Nanoscale compositional banding in binary thin films produced by ion-assisted deposition

During the ion-assisted deposition of a binary material, the ion beam can induce the formation of nanoscale ripples on the surface of the growing thin film and compositional banding within its bulk. We demonstrate that this remains true even if the curvature dependence of the sputter yields and ballistic mass redistribution are negligible, and the two atomic species are completely miscible. The concentration of the species with the lower of the two sputter yields is higher at the crests of the ripples than at their troughs. Depending on the angles of incidence of the two atomic species, the incident flux of atoms with the higher sputter yield can either stabilize or destabilize the initially flat surface of the thin film.

Investigation of ion beam induced nanopattern formation near the threshold energy

The nanoscale ripple formation on mica surface is studied at off-normal ion incidence angles θ under Ar+ bombardment at energies E close to or below the threshold energy for physical sputtering. A phase diagram for domains of pattern formation is presented as a function of θ and E, which shows the stability/instability bifurcation angle close to 40°. The instability grows as ripple-like structures at lower angles, while at grazing angles the pattern emerges as dense array of needles. The results support the pure mass redistribution based paradigm for surface patterning as the erosion due to sputtering essentially tends to zero.

Morphology evolution of fused silica surface during ion beam figuring of high-slope optical components

Ultra-precision and ultra-smooth surfaces are vitally important for some high performance optical systems. Ion beam figuring (IBF) is a well-established, highly deterministic method for the final precision figuring of extremely high quality optical surfaces, whereas ion sputtering induced smoothing, or roughening for nanoscale surface morphology, strongly depends on the processing conditions. Usually, an improper machining method would arouse the production of nanoscale patterns leading to the coarsening of the optical surface. In this paper, the morphology evolution mechanism on a fused silica surface during IBF of high-slope optical components has been investigated by means of atomic force microscopy. Figuring experiments are implemented on two convex spherical surfaces by using different IBF methods. Both of their surface errors are rapidly reduced to 1.2 nm root mean square (RMS) after removing similar deep material, but their surfaces are characterized with obviously different nanoscale morphologies. The experimental results indicate that the ion incidence angle dominates the microscopic morphology during the IBF process. At near-normal incidence, fused silica achieves an ultra-smooth surface with an RMS roughness value R(q) down to 0.1 nm, whereas nanoscale ripple patterns are observed at a large incidence angle with an R(q) value increasing to more than 0.9 nm. Additionally, the difference of incidence angles on various machined areas would influence the uniformity of surface quality, resulting from the interplay between the smoothing and roughening effects induced by ion sputtering.

Competing Striped Structures in La2CuO4+y

High temperature superconductivity emerges in unique materials, like cuprates, that belong to the class of heterostructures at atomic limit, made of a superlattice of superconducting atomic layers intercalated by spacer layers. The physical properties of a strongly correlated electronic system, emerge from the competition between different phases with a resulting inhomogeneity from nanoscale to micron scale. Here, we focus on the spatial arrangements of two types of structural defects in the cuprate La2CuO4+y : (i) the local lattice distortions in the CuO2 active layers and (ii) the lattice distortions around the charged chemical dopants in the spacer layers. We use a new advanced microscopy method: scanning nano X-ray diffraction (nXRD). We show here that local lattice distortions form incommensurate nanoscale ripples spatially anticorrelated with puddles of self-organized chemical dopants in the spacer layers.

Crossover in the surface anisotropy contributions of ferromagnetic films on rippled Si surfaces

We investigate the influence of a nanoscale periodic ripple morphology on the structure and magnetocrystalline anisotropy of thin ferromagnetic (FM) Ni${}_{81}$Fe${}_{19}$, Co, and Fe films. The ripples are created by ion beam erosion of the Si substrate. The periodic ripple structures induce a uniaxial magnetic anisotropy (UMA) in the FM films as confirmed by ferromagnetic resonance and magneto-optical Kerr effect measurements. The thickness dependence of the UMA reveals an abrupt transition. For a thin film regime there is a corrugated alignment of the magnetic moments and above a critical thickness one has dipolar interactions due to the sinusoidal surface modulations.

Effect of stationary femtosecond laser irradiation on substructures’ formation on a mold stainless steel surface

Nano-scale and micro-scale laser-induced periodic surface structures (ripples), pores and splash-like structures with different formation thresholds fluence have been observed after stationary multiple linearly polarized femtosecond laser irradiations on a molding stainless steel. Nano-scale structures exhibit a formation threshold fluence lower than that of micro-scale ripples. When increasing both power density and number of pulses, nano-scale ripples are only sensitive to the latter, showing a 15% decrease of their spatial period in the range of used values. Micro-scale ripples respond to these two laser parameters by an increase followed by a decrease of their spatial period. The two types of ripples exhibit behavior differences that show that micro- and nano-scale ripples are the results of two distinct mechanisms.

Local Thermomechanical Analysis of a Microphase-Separated Thin Lamellar PS-b-PEO Film

We use atomic force microscopy (AFM) and hot tip AFM (HT-AFM) to thermophysically characterize a 30 nm thick film of poly(styrene-block-ethylene oxide), PS-b-PEO, and to modify its lamellar patterns having spacing of 39 ± 3 nm. AFM tip scans of the polymer film induce either abrasive surface patterns or nanoscale ripples, which depend upon the tip force, temperature, and number of scans. The evolution of the lamellar patterns is explained by the polymer film molecular structure and mode I crack propagation in the polymer combined with the stick-and-slip behavior of the AFM tip. The HT-AFM measurements at various tip-sample temperatures and scanning speeds yield several thermophysical quantities: the PEO melting temperature of 54 ± 12 °C, the PS glass transition temperature of 54 ± 12 °C, the PS-b-PEO specific heat of 3.6 ± 2.7 J g(-1) K(-1), the PEO melting enthalpy of 111 ± 88 J g(-1), and the free energy of Helmholtz for PEO unfolding (and melting) of 10(-20) J nm(-2). These quantities are obtained for PS-b-PEO volumes of 30,000 nm(3), which correspond to 30 ag of the polymer.

Tailoring resisitivity anisotropy of nanorippled metal films: Electrons surfing on gold waves

By using self-organized patterning of polycrystalline Au electrodes, we demonstrate that the electrical resistivity of the film can be forced to become strongly anisotropic by tailoring a nanoscale ripple undulation at the metal-vacuum interface. Electrons traveling orthogonally to the nanoripples are affected by a strong increase in resistivity, while when electrons travel parallel to the undulations of the rippled film, the resistivity is almost unaffected and even shows a slight reduction. The observations are quantitatively interpreted within the semiclassical size-effect regime in terms of an increase of surface scattering from the large-scale corrugations and of a reduction of diffuse scattering from atomic-scale roughness.

Propagation of nanoscale ripples on ion-irradiated surfaces

The irradiation of glass surfaces by 30keV Ga+ ions under oblique incidence angles results in the development of “ripple”-like nanostructures. The evolution of these ripples was monitored in situ in the ion fluence range from 3×1016 to 1×1018 Ga+ions/cm2 using a focused ion beam system which incorporates a scanning electron microscope. With increasing fluence, the ripples were found to propagate across the surface in a direction which coincides with the projection of the ion beam’s incident direction onto the surface. The propagation velocity was determined to be constant over the investigated fluence regime and to amount to 77±1.8nm/1017ionscm−2. Ex situ atomic force microscopy was employed to derive the post-mortem wavelength and amplitude of the ripple structures and their values were 450±43 and 127±23nm, respectively.

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.

Effect of nanoscale ripples on the formation of ZnO quantum dots

Nano-scale ripples on Si (100) substrates were prepared by argon ion beam sputtering at 6, 8, and 10 keV with an ion fluence of 5 × 1017 ions/cm2. The spatial wavelength of the nano-scale ripple increases as the ion beam energy increases, regardless of ion beam incident angles, indicating that ion beam induced diffusion is the dominant diffusion mechanism. ZnO quantum dots (QDs) with diameters less than 20 nm and heights less than 4 nm can be prepared over the temperature range from 200 ∼ 300 °C by reactive ion beam sputter deposition. Compared with ZnO QDs deposited on Si substrates without nano-scale ripple, the ion-beam textured substrate provides a wider processing window, improved dot size, and increased QD density.

Thickness dependence of the magnetic properties of ripple-patterned Fe/MgO(001) films

Grazing incidence Xe${}^{+}$ ion sputtering was used to create a nanoscale ripple pattern on a thin Fe film, epitaxially grown on MgO(001). The Fe film has a thickness gradient of 0--20 nm and a ripple height of about 3 nm, giving rise to a transition from a continuous film to separated nanorods with decreasing film thickness. This allowed the investigation of the competition between the uniaxial and biaxial anisotropy of the irradiated sample as a function of thickness. From magneto-optical Kerr effect measurements, we determine accurately the cubic magnetocrystalline anisotropy and the uniaxial anisotropy that originates from the ripple pattern using a coherent rotation model. Our results show that the uniaxial anisotropy strength increases, whereas the contribution of the biaxial crystal anisotropy decreases, when going from the continuous film to the nanorod structures.

Growth of ZnO quantum dots on Si nano ripples

Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~ 70 to ~ 150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~ 30 nm and heights from 2 to ~ 4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~ 4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented.